This invention relates to a method of refining silicon tetrafluoride gas which contains oxygen-containing silicofluoride(s) such as fluorosiloxane and/or fluorosilanol as impurity matter.
High purity silicon tetrafluoride gas is useful for the preparation of amorphous silicon semiconductor which is expected as an advantageous material for various electronic devices including photovoltaic cell elements.
As is well known, silicon tetrafluoride gas readily reacts with water in liquid state to form hexafluorosilicic acid and gel-like silica as represented by the following equation (1). Furthermore, silicon tetrafluoride gas reacts with moisture in the atmosphere and even with a trace amount of water adsorbed on a metal or glass surface, or with the combined water in a clay-like mineral material such as zeolite or kaolin, to form hexafluorodisiloxane as represented by the following equation (2). EQU 3SiF.sub.4 +2H.sub.2 O.fwdarw.2H.sub.2 SiF.sub.6 +SiO.sub.2 ( 1) EQU 2SiF.sub.4 +H.sub.2 O.fwdarw.(SiF.sub.3).sub.2 O+2HF (2)
Therefore, it is almost inevitable that silicon tetrafluoride gas prepared by reaction between silica sand, silica gel or a silicate with hydrogen fluoride or hydrofluoric acid contains a certain amount of hexafluorodisiloxane as impurity matter. Besides, the result of mass spectrometry of silicon tetrafluoride gas often indicates the presence of trifluorosilanol SiF.sub.3 OH too.
In the production of amorphous silicon by using silicon tetrafluoride gas by a glow discharge method for example, the presence of any oxygen-containing silicofluoride having either Si-SO bond or Si-O-Si bond in the silicon tetrafluoride gas is liable to result in the intrusion of Si-0-Si bond into the intended Si-Si network structure with detrimental influences on the properties of the obtained amorphous silicon as a semiconductor material.